JSP跳转到Servlet出现404错误

[PLATINUM_II]

JSP：

<div class="fbneirong">
<form class="am-form" action="ChangeBookServlet"method="post" >
<div class="am-form-group am-cf">
<div class="zuo">编号：</div>
<div class="you">
<input type="text" class="am-input-sm" id="doc-ipt-pwd-1" name="book_id" placeholder="${bookList[0].book_id}" readonly="readonly">
</div>
</div>


Servlet：

package com.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.bean.Book;
import com.dao.BookDao;

/**
* Servlet implementation class ChangeBookServlet
*/
@WebServlet("/ChangeBookServlet")
public class [color=#FFCC00]ChangeBookServlet[/color] extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public ChangeBookServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doPost(request, response);
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");

String id = request.getParameter("book_id");
int book_id = Integer.parseInt(id);
System.out.println(book_id);
String book_name = request.getParameter("book_name");
String book_writer = request.getParameter("book_writer");
float book_price = Float.parseFloat(request.getParameter("book_price"));
String book_img = request.getParameter("book_img");
Book book = new Book(book_id, book_name, book_writer, book_price, book_img);
BookDao.updateBook(book);
}

}


在jsp的action中加了/也不行，写全路径也不行。web.xml中什么都没写，但是添加删除查找都可以，就是这个Servlet跳转不到。

[PLATINUM_II]

[PLATINUM_II]

我把Servlet换成其他的都可以跳转，就是这个不行。

[PLATINUM_II]

重启电脑后404变为500，原因是 placeholder不是默认参数，servlet获取不到值，将 placeholder改为value就好了。